Abstract: Experimental results from atmospheric, solar, and reactor neutrino sources have identified neutrino oscillations and have established beyond doubt that neutrinos have rest masses. Yet neutrino oscillations are only sensitive to the differences of squared masses of different neutrino mass eigenstates. The emerging alternative direct neutrino mass measurement method based on calorimetric electron capture spectroscopy (ECS) of 163Ho will be discussed.
The ECS method is similar to regular beta decay method in that the shape of the energy spectrum near the kinematic end point is sensitive to neutrino mass (similar fraction of events in the relevant endpoint region). By embedding 163Ho inside low temperature micro calorimeters, one can capture, sum and measure all the decay energy except for that of the escaping neutrino. Because the total nuclear decay energy Q of 163Ho is low ( Q < 3 keV), the spectroscopic resolution and the number of events near the endpoint are now matched by the technological capabilities for high-resolution sensors and large sensors arrays.
The NUMECS collaboration has over the last two years demonstrated proton-beam based production of high isotopic purity 163Ho on the 0.1 micro gram scale; chemical purification and embedding into sensors; successfully developed dedicated single channel ECS sensors and has shown that the required energy resolution can be achieved for the surrogate ECS decay of 55Fe.